In general, advance of technology results in swift development of rehabilitation robot; currently, rehabilitation robot can already help physical therapists perform high-intensity training for patients. Therefore, a lot of manpower and time are saved. In addition, rehabilitation robot can not only train patients by conventional rehabilitation training, but also can train the patients by interactive rehabilitation training. In other words, the stiffness of the joints of a robot is adjustable. When the muscle strength of a patent is insufficient, the stiffness of the joints of the robot should increase to help the patient in training; on the contrary, when the muscle strength of the patent is sufficient, the stiffness of the joints of the robot should decrease for the patient to more effectively train his/her muscle. For the purpose of adjusting the stiffness of the joints of the robot, it is necessary to install an elastic actuator on the robot.